Is there a way to calculate how much water a dog needs a day based on weight?

As a general rule I would say most dogs need about an ounce of fluids per pound of body weight per day.

Dogs typically need 1/2 to 1 ounce of water per pound of body weight every day! You can life some skin at the back of your dogs beck and release it to see if your dog is well hydrated. If the dog is well hydrated, the skin will go quickly right back into place.